The present invention relates to a bubble detecting apparatus for detecting air bubbles produced in or mixed into a liquid transport tube for use in a medical infusion device for mainly infusing medicinal liquid into a human body, for example, a peristaltic type infusion pump in which the medicinal liquid is intermittently fed by repeating a predetermined liquid feed process.
Generally, in known liquid transport systems employing liquid infusion pumps, a vial is attached to an upper end of a liquid transport tube assembly from an upper portion of which a drip chamber, a check valve and a Y-shaped injection set are sequentially provided. Meanwhile, an infusion pump is mounted on a lower portion of the liquid transport tube assembly and is constituted by a pumping mechanism, an internal pressure sensor and a bubble detecting apparatus. The bubble detecting apparatus detects downwardly flowing air bubbles produced in or mixed into the liquid transport tube. Operation of the pumping mechanism is stopped in response to a bubble detection signal of the bubble detecting apparatus, thereby eliminating such a phenomenon that a human body is set in a dangerous condition through injection of air bubbles into the human body.
As shown in FIG. 1, the known bubble detecting apparatus includes two ultrasonic sensors 1 and 2. The ultrasonic sensor 1 is constituted by an ultrasonic oscillator 1A and a receiver 1B disposed at opposed locations on an outer face of a liquid transport tube 3. Likewise, the ultrasonic sensor 2 is constituted by an ultrasonic oscillator 2A and a receiver 2B disposed at opposed locations on the outer face of the liquid transport tube 3. The ultrasonic sensors 1 and 2 are axially spaced a distance X from each other. When an air bubble 5 produced in or mixed into medicinal liquid 4 in the liquid transport tube 3 flows in between the ultrasonic sensors 1 and 2 as shown, both ultrasonic waves outputted by the ultrasonic oscillators 1A and 2A, respectively are intercepted by the air bubble 5 and therefore, are not received by the receivers 1B and 2B, whereby it is decided that the air bubble 5 exists.
Meanwhile, in the known bubble detecting apparatus, since presence of the air bubble is detected at the time when both ultrasonic waves from the ultrasonic oscillators 1A and 1B are not received by the receivers 1B and 2B, respectively, the air bubble 5 detectable by the known bubble detecting apparatus is so restricted, in size, as to be larger than the distance X between the ultrasonic sensors 1A and 1B. However, this known bubble detecting apparatus cannot be used in the case where the air bubble 5 has a size smaller than the distance X according to service temperature of the infusion pump, kinds of the medicinal liquid, etc. Even if the air bubble 5 is of a quite small size, injection of the air bubble 5 into the human body may set the human body in a dangerous condition and in some cases, destroys life.